Shapekeys before or after normal mapping?
First of all, is your character using a Mirror Modifier to generate symmetry? If it is, you should apply it first before doing your UV mapping, texturing, rigging, and shape-keying. (Symmetrical UV maps are OK in some special cases, but definitely avoid them if you will need to use a Normal Map.)

As for creating a low-poly mesh from your high-poly one, check this technique out. It might seem a bit confusing if you're just starting out, but understanding it can save you a lot of time when creating your low-poly meshes.

Your Normal Map will not change with the Shape Keys, nor should it need to unless you're trying to do something overly complex. 

Use the high-poly mesh to bake your maps onto the low-poly mesh. Keep both handy, and only delete your high-poly sculpt once you're done baking textures. Keep a version of your project saved that still contains your final sculpt in case you need to re-bake later.